As a member for a shoe sole, a soft material which is a crosslinked product of a natural rubber, a synthetic rubber, or the like is often used from the aspect of grip performance or abrasion resistance. A shoe roughly comprises an upper (a part covering a feet), a midsole (a part showing a cushion property), and a shoe sole (a part contacting with the ground). The shoe sole is further divided into a stud (a part projecting from a shoe sole and firmly contacting with the ground) and a sole (a part other than the stud of the shoe sole). A rubber is generally useful for the stud from the aspect of grip performance or abrasion resistance, and a thermoplastic elastomer is generally useful for the sole from the aspect of spring performance.
Heretofore, with respect to the adhesion between the stud and the sole, since it was difficult to ensure a sufficient adhesion strength by an ordinary adhesion method due to a small adhesion area thereof, both of the stud and the sole were usually made of rubber or thermoplastic elastomer in many cases. However, when both of the stud and the sole are made of rubber, the resulting shoe increases in weight due to a large specific gravity of the rubber, which causes a problem that the spring performance is hardly obtained. Whereas, when both of the stud and the sole are made of thermoplastic elastomer, the resulting shoe has a disadvantage for fusion bonding in an artificial turf ground, a gymnasium, or others due to frictional heat or for easy slipping on a wet road surface.
Accordingly, in order to mold (or form) a composite in which a stud is joined to a sole at a sufficient adhesion strength even when the materials for both stud and sole are different from each other, various methods have been attempted. A composite molding by direct adhesion without an adhesive is one of effective methods, by which a shoe sole comprising a stud and a sole joined thereto at a sufficient adhesion strength can be obtained even when the stud comprises a rubber and the sole comprises a thermoplastic elastomer.
For example, Japanese Patent Application Laid-Open Publication No. 8-294933 (JP-8-294933A, Patent Document 1) discloses a process for multi-molding a thermoplastic resin (I) selected from a polyurethane (for example, a polyether urethane, a polyester urethane) and a polyamide-based polymer (for example, a polymer having a polyamide block and a polyether block, such as a polyether ester amide) over a rubber (II), the process comprising halogenating a surface of the rubber and then multi-molding the thermoplastic resin by compression molding or injection molding. This document discloses that the composite material obtained by the process is used for production of sports shoes which have a polyamide elastomer shoe sole having a rubber stud or rubber heel fixed thereon for preventing the shoe sole from abrasion.
Moreover, Japanese Patent Application Laid-Open Publication No. 8-511741 (JP-8-511741A, Patent Document 2) discloses a composite structure comprising a vulcanized elastomer containing a carboxylic acid group, and a thermoplastic polymer containing a block (for example, a polyether ester, a polyurethane ether, a polyurethane ester, and a polyurethane ether ester), wherein the elastomer is directly associated with the polymer. Moreover, this document discloses that the composite structure is useful as a shoe sole for sports shoes.
In composite molding of a rubber and a thermoplastic elastomer, however, a process with heating or pressurizing (such as injection molding or compression molding) is usually adopted, and in particular, defective products due to deformation of the thermoplastic elastomer side becomes a problem. On the contrary, easing of the heating or pressurizing condition causes a problem, such as prolongation of the vulcanizing time of the rubber or imperfect adhesion between the rubber and the thermoplastic elastomer. These documents are silent on a method for solving such a problem. In addition, according to the process described in the Patent Document 1, the pre-treating step, i.e., halogenation of the surface of the rubber, is essential, which is disadvantage when the processing time is desirable to be shortened.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2000-41702 (JP-2000-41702A, Patent Document 3) discloses a shoe sole having a three-layered structure, which is produced by simultaneously integrating an outsole member and a shoe sole body by injection molding a thermoplastic elastomer for the shoe sole body to the outsole member composed of an outsole design part of a synthetic rubber and a thermoplastic elastomer sheet joined and integrated thereto. This document discloses that the shoe sole can be produced by die-cutting a kneaded sheet of an uncrosslinked synthetic rubber to give a member for an outsole design, placing the member in a depressed portion for outsole design of a shoe sole metal mold (or a shoe sole die) for pre-molding, applying pressure and heat to the member, then opening the metal mold at an initial stage of the crosslinking in the member for outsole design, installing a thermoplastic elastomer sheet having a shape practically corresponding to an outer shape of the depressed portion for outsole design, applying pressure and heat again to the member to form an outsole member in which the design part of the synthetic rubber and the thermoplastic elastomer sheet are integrated, trimming the outsole member into the shape of a depressed portion for design of an injection molding metal mold for outsole, clamping the metal mold, then installing the outsole member so as to correspond to the depressed portion for design of the injection molding metal mold for outsole, and then injection molding a thermoplastic elastomer for forming the shoe sole body.
However, this document is also silent on a method for solving the problem of composite molding as described above. Moreover, according to this document, it is necessary for joining and integrating of the outsole design part and the outsole member to stop the crosslinking step of the synthetic rubber, open the metal mold, install the thermoplastic elastomer sheet, and then close the metal mold. The set of steps become problems when the increase in operating efficiency is desired. Furthermore, in injection-molding the thermoplastic elastomer for forming the shoe sole body to the outsole member, heat-fusing (fusing integration) requires use of substantially the same kind of resin as the resin for outsole member and the thermoplastic elastomer. Thus the combination of the resin materials is limited.